Retractable assemblies are available from the group of firms, Endress+Hauser, in a large number of variants, for example, that bearing the designation “Cleanfit H CPA475”.
Retractable assemblies are widely used in analytical measurements technology. They serve for introducing and removing probes to and from a process and its media without process interruption. The probes are secured to an immersion tube and, by means of a drive, moved manually or automatically, for example, pneumatically, axially between a process position and a service position. These events happen within a certain timing cycle or as a function of other determinable or measured parameters.
Probes in the sense of this invention comprise probes with at least one accommodation for at least one sensor for measuring one or more physical or chemical, process variables.
The fields of use of retractable assemblies for measuring physical or chemical, process variables of a medium, e.g. a fluid, especially a liquid, in process technology are many. Sensors are used for determining the process variables, wherein the sensors can be, for example, pH-sensors, conductivity sensors, optical or electrochemical sensors for determining a concentration of a substance contained in the medium to be monitored, e.g. O2, CO2, certain types of ions, organic compounds, etc.
The assembly, e.g. retractable assembly, represents an important part of the investment- and operating costs of a measuring point. The assembly accommodating the sensor is always contacting the medium and, thus, highly safety relevant. The assembly is the connection to the process. A lack of sealing and the then necessary repair of the assembly lead to process interruption. Dependent on the process medium, a defective assembly can endanger humans and the environment.
If retractable assemblies are used for accommodating the sensor for determining at least one process variable, the sensor can be checked, calibrated, cleaned and/or replaced in the service position, wherein the sensor is, in such case, located in a treatment chamber arranged in the housing of the retractable assembly.
“Above” and related terms mean, in the sense of this invention, remote from the medium. “Below” and related terms mean, in the sense this invention, toward the medium.
The reaching of the service position is recognized by the striking of the immersion tube on an upper stop on the housing remote from the medium. In the case of automatic retractable assemblies, there are variants, in the case of which the achieved position of the immersion tube in the service position is reported to a control unit by a reporter using an end position switch; see, in this connection, the already mentioned CPA475.
In the service position, the solid lower end of the immersion tube seals off from the process (the medium). Thus, there is a seal between the process containment and the housing interior and therewith the sensor space. Only in this position, is it permitted that the assembly be opened, so that the sensor can be taken out, without, in such case, suffering the escape of process medium.
A secure sealing off of the process, thus the closing the process opening, is very important, since otherwise, in the case of opening the assembly, or the screwing out of the sensor for service purposes, process medium can escape through the immersion tube. In many cases, the process medium is under pressure. It can be hot, poisonous or corrosive. An unintended opening is highly endangering for humans and environment. For repair, the process must be stopped and the area, in given cases, cleaned and even decontaminated.
In the case of previously known retractable assemblies, the partitioning off of the process is only assumed from the fact that the immersion tube has retracted completely into the assembly, thus the upper edge of the immersion tube has reached the upper limit or the piston of the pneumatic drive has reached the end position, and, thus, the lower end of the immersion tube has as closure element sealed off the process. The upper position reporter detects only the raised position of the immersion tube.
In the case, in which the immersion tube or the thereon located closure element has broken off, indeed, an “immersion tube in service position” is detected, while, nevertheless, the lower region of the immersion tube is missing and the opening to the process is still open. If, in this case, the assembly is opened for sensor service, process medium rises through the immersion tube into the environment. This represents a great danger that damage will occur.